In general, a motor vehicle exhaust system is mounted at the outlet of an exhaust manifold of the engine and is suspended from the body of the vehicle. The presence of a decoupling hose enables the exhaust system to accommodate movements of the engine associated with vertical accelerations, sudden changes in speed, thermal expansion, assembly tolerances, . . . , and provides the flexibility needed to decouple engine vibrations from the exhaust line and from the body, thereby improving comfort within the vehicle cabin.
Document FR-A-2 796 416 discloses a decoupling hose which comprises, in particular, an inner portion suitable for channeling the flow of exhaust gases, an outer portion in the form of a continuous leaktight sleeve having a flexible central zone and two rigid endpieces, and an intermediate portion in the form of thermal lagging which extends substantially over the full length of the outer sleeve. The two metal endpieces serve essentially to protect the flexible central zone of the outer sleeve of the hose from high temperatures by keeping said central zone as far as possible away from hot zones where the hose is connected to the exhaust system.
Furthermore, any hose mounted on a vehicle is subjected to deformation in several directions and in particular to transverse displacements in shear which can reach amplitudes of ±50 millimeters (mm) on large vehicles. During endurance tests performed on a hose of the above-specified type, the Applicant has found that the thermal lagging becomes damaged by each endpiece in the vicinity of its peripheral end edge that is connected to the central zone of the outer sleeve. During large-amplitude movements in shear, the lagging is locally compressed and said end peripheral edges of the endpieces then cut into the lagging in the compressed zone.
When testing on vehicles fitted with hoses of a length lying in the range 150 mm to 200 mm, maximum amplitudes of movements in shear have been found of the order of ±15 mm to ±20 mm, and after running 10,000 kilometers (km) the above-mentioned wear phenomenon can be observed. Furthermore, simulation bench tests consisting in imposing shear strains to the hose with amplitudes of ±12 mm in combination with traction and compression strains of the order of 3 mm have led to the above-mentioned wear phenomenon being observed after 500,000 cycles.
Analysis of those various tests has shown that the lagging becomes worn as a result of the inner portion of the hose deforming over a length that is longer than the length of the flexible central zone of its outer sleeve.